Electrical engineering & Electromechanics (Jun 2019)
TECHNOLOGICAL PARAMETERS OF THE COOLING MODE OF POLYMER INSULATION OF POWER CABLES
Abstract
Introduction. The cooling mode of polymer insulation after application to the extruder is one of the main factors determining cable performance. Theoretically, it is ideal to cool the insulation when the temperature of the cooling medium is equal to the melting point of the insulation material: in this case, the probability of formation of voids in the insulation is less. The cooling process is usually not subject to stringent requirements, since most insulating materials allow for quite sharp cooling. The exception is polyethylene, which requires gradual cooling. When the insulation is cooled in a cooling bath, the temperature decrease starts from the surface. In this regard, the cooling of the insulation of polyethylene is carried out in steps to a temperature at which the cooled extruded insulation will not be deformed or damaged on the receiving drum. Polyethylene is characterized by a large value of thermal expansion coefficient, the maximum value of which is in the temperature range (90-125) °C. As a result, there is an uneven reduction in the volume of the upper and inner insulation layers, especially for cables with a considerable insulation thickness. The rapid cooling of polyethylene leads to the formation of cracks, air inclusions both between the insulation and the conductive core, and in the layers located near the core. Purpose. The substantiation of the technological parameters of the cooling mode of power cables based on the calculation of the thermal equivalent circuit of a conductive core insulated with polyethylene in transient thermal mode. Methodology. The calculation of the temperature distribution in the thickness of extruded polyethylene insulation at different points in time, depending on the temperature of the cooling water, is made by the method of electrothermal analogies. There is a transition from the thermal equivalent circuit of power cables to the equivalent circuit of the discrete resistive equivalent circuit method, which is calculated using the nodal potential method. As a result of solving a three-diagonal system of linear algebraic equations by sweeping and finding at each discretization step (time step) thermal power fluxes in the branches of the thermal equivalent circuit, the temperature in the thermal capacitances determines the temperature in each insulation layer. Practical value. The duration of the transition process, corresponding to the achievement of the same temperature throughout the thickness of the insulation, can be considered as a criterion in determining the length of the cooling bath sections depending on the extrusion (reception) rate.
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